It goes by different names—steering knuckle, upright, wheel carrier—and varies with suspension design and drive layout, but this part is always the nexus
of a car's front axle. The knuckle is where wheel meets suspension meets steering. In the case of front- or all-wheel-drive cars, this is also where power
enters the mix. The wheel mounts to the outside, via a hub or a spindle, and the tie rod and suspension connect inboard via mounts that allow the knuckle
to pivot (i.e., steer the car).

Executive Summary:
It holds the front wheel on the car and lets you change direction.